The field of the invention relates to identification and location of faults in electrical power transmission lines.
The ability to accurately determine the location of faults on power system lines and estimate the approximate fault impedance are important as they facilitate fast dispatch of the field crews, faster inspection and shorter repair times all leading to faster restoration of the affected transmission line. At the same time accurate fault location is a technical challenge primarily because the fault location estimation is done based on very limited amount of information gathered at the line terminals only. Problems which must be overcome include finite modeling accuracy of transmission lines, instrument measurement errors, errors in the estimation of parameters of the line and system models, coupling to adjacent power system transmission lines, unknown and often non-linear fault resistance, finite duration of faults resulting in short time window of available data.
Fault location is commonly performed as an adjunct to the functioning of distance-based power system protective relays. The most common approaches use voltage and current measurements from a single line terminal to estimate the fault location using various assumptions and approximations. Such approaches are referred to as single-ended methods and are not very accurate. The lack of absolute accuracy is primarily a result of having more unknowns than equations that could be derived from the line and system model based on measurements from one end of the line. As a result assumptions are made. Various assumptions yield various single-ended fault location methods. When the assumptions are satisfied in a given fault situation, the fault location result is accurate. If the assumptions are not satisfied, an inherent, sometimes very significant, error of the method will occur.
Fault location systems that utilize information from more than one line terminals are referred to as multi-ended fault locators. A multi-ended fault locator eliminates the key weakness of a single-ended approach, but requires communication channels to rely data from geographically dispersed line terminals to a single location where the actual fault location calculations are performed. Some multi-ended fault location methods also require synchronization of the data between the line terminals. These two requirements make the multi-ended fault location methods difficult to implement. U.S. Pat. No. 6,256,592, for example, describes a multi-ended system for locating a fault on a power line using the magnitude value of the negative sequence current and the magnitude and angle values of the negative sequence impedance at the time the fault occurs. The magnitude and angle information is transmitted between two terminals of the multi-ended system so that the fault location point can be determined from the information. U.S. Pat. No. 6,256,592 uses the negative sequence current information to produce results in near real time by reducing the amount of data that must be transmitted between terminals. U.S. Pat. No. 6,879,917 uses positive- or negative-sequence currents and voltages to locate faults. Most fault types are covered by the negative-sequence method of the patent. Three-phase balanced faults do not produce any negative-sequence signals rendering the negative-sequence method of U.S. Pat. No. 6,879,917 useless. Therefore the said patented method adds the positive-sequence based equations to eliminate this weakness. As a result, two sets of calculations must be run in parallel, or a coarse fault type identification must be performed.
The need for fault type identification is a weakness for real-time systems with limited communication bandwidth. The remote portion of the locator needs to send both negative- and positive-sequence based signals, or the two portions of the locator must work flawlessly in terms of fault type identification. If one portion sends the negative-sequence based information, while the other portion combines it with the positive-sequence based information, significant errors in the fault distance estimate will occur.
A typical single- or multi-ended fault locator requires knowledge of the fault type, i.e. which and how many conductors are involved in the fault, knowledge of the mutual coupling to adjacent lines located on the same towers or in close proximity, and some other auxiliary information. These extra factors are found through separate procedures, and if delivered to the main fault location procedure with errors, they will impact the overall fault location accuracy.
A need still exists for other methods of accurately determining fault position on a power transmission line, which can further reduce error and produce results quickly following a fault. For multi-ended systems working in real or near-real time, such as locators integrated with protection relays, it is important to limit the bandwidth requirements for communications, and in particular, the amount of information that needs to be sent between different terminals of the transmission line.